The present invention relates to a driver circuit for a two-dimensional, solid-state image sensor, and particularly to a driver circuit for a two-dimensional CCD image sensor operating in a field storage mode.
Two-dimensional CCD image sensors operating in a field storage mode are conventionally used in color TV cameras. In the field storage mode, transmissions for signal charges from photodiodes are carried out for each field. Assume that a two-dimensional CCD image sensor formed of 490 pixels in the vertical direction is applied to an NTSC color TV camera. In such a TV camera, 245 sets of vertical transmission registers are provided for the CCD image sensor, thereby achieving simultaneous readout of signal charges from the 490 photodiodes for each field. In this case, the above readout is performed such that the signal charges from two photodiodes adjacent in the vertical direction are added to each other.
According to a conventional driver circuit for a two-dimensional, CCD image sensor operating in the field storage mode, the interval from the time just after completing the final line shift to the time when the next field shift is effected varies with the change of fields, and the amplitude of field shifting pulses varies for each field. This factor gives rise to and invites the occurrence of flickering in the obtained TV image.
Flickering of the TV image occurs for the following reason. The vertical transmission electrode of a CCD image sensor has distributed resistances and distributed capacitances with respect to the substrate of the CCD semiconductor chip. If the time interval from the final line shift to the next field shift varies with each field change, and if this time interval is not sufficiently longer than one corresponding to the time constant of said distributed resistances and distributed capacitances, then the potential of the field shift pulse obtained just after completing the final line shift is changed for each field. From this, the potential of particular photodiodes in the CCD varies slightly for each field. Under this condition, since the subsequent readout is started from the initial point of the next field, the actualy obtained initial potential of the photodiode deviates slightly from the prescribed initial potential. Such an initial potential deviation will cause a material disadvantage, i.e., the occurrence of flickering, even if the value of this potential deviation is less than 1/100 of the prescribed initial potential.
To be concrete, when the initial potential of the photodiode is adjusted for an even field and the signal charge is read out in an odd field, DC charges corresponding to the initial potential deviation are excessively read out, thereby causing said flickering. Conversely, if the initial potential of the photodiode is adjusted for the odd field and the signal charge is read out in the even field, DC charges coresponding to the initial potential deviation are lacking in the readout signal charge, such that this too causes flickering.
Flickering is liable to occur whenever the resistivity of the CCD substrate is relatively high and/or the time constant formed by the distributed resistances and distributed capacitances of the vertical transmission electrode with respect to the CCD substrate is relatively large. From this, it can be deduced that flickering is highly likely to occur when the CCD has a P-well structure or VOD (Vertical Overflow Drain) structure.